1. Field of the Invention
The present invention relates generally to an FM discriminator and more particularly to an FM discriminator with AFC signal producing circuits which will produce AFC signals opposite in polarity.
2. Description of the Prior Art
In the prior art FM receiver, the output signal from its FM discriminator circuit is supplied to a low pass filter which produces a DC voltage or AFC voltage whose polarity and level varies in accordance with the drift of an intermediate frequency f.sub.i. The AFC voltage is then supplied to a variable capacitance diode which is connected in parallel to the variable capacitor and coil of a local oscillator circuit for determining the oscillation frequency of the local oscillator circuit to be automatically compensated.
In this case, however, the AFC voltage is varied in polarity and level in accordance with the intermediate frequency f.sub.i so that, in order to place the variable capacitance diode in a reverse-biased state irrespective of the AFC voltage, it is necessary to employ a constant voltage diode for supplying a reverse bias voltage to the variable capacitance diode.
For example, in Japan an FM receiver employs a system wherein the local oscillation frequency is lower than the received frequency (a lower heterodyne system) while in, for example, the USA, an FM receiver employs a system wherein the local oscillation frequency is higher than the received frequency (an upper heterodyne system). Therefore, with respect to the intermediate frequency f.sub.i, it is required that the polarity and level of the AFC voltage of an FM receiver for the USA be reversed from the AFC voltage of an FM receiver for Japan. For this reason, even in the same discriminator circuits, it is necessary that the polarities of a pair of diodes for ratio detecting in the FM receivers for Japan are opposite to those for the USA. This creates an obstacle for mass-production of FM receivers.
In general, since the front end and intermediate frequency amplifier and discriminator circuits are provided on separate printed boards in an FM receiver, it is preferable that the intermediate frequency amplifier and discriminator are the same for both the lower heterodyne and upper heterodyne systems. However, due to the fact that the polarities of a pair of diodes for ratio detection are opposite for the two systems, the intermediate frequency amplifier and discriminator can be made the same. Hence, mass-production is inhibited.